1. Field of the Invention
The present invention relates to a sintered type negative cadmium electrode for an alkaline storage cell and to a method of manufacturing the same.
2. Description of the Prior Art
Generally, an electrode employed in an alkaline storage cell is manufactured by the following two methods. The first method is an unsintering method wherein, as disclosed in Japanese Patent Publication Kokai No. 56-82573, the electrode is obtained by kneading powder of an active material with a binder to form a paste, coating the paste on surfaces of a conductive plate and drying the paste. The second method is a sintering method wherein, as disclosed in Japanese Patent Publication Kokai No. 51-18834, the electrode is filled with a desired amount of active material by repeating an active material impregnating process several times. In this process, a porous nickel sintered plaque is impregnated with cadmium nitrate solution, and thereafter the product is subjected to an alkali treatment, rinsing and drying.
The former electrode employing polyvinyl alcohol as the binder is heat treated at a temperature of 120.degree.-220.degree. C. to prevent the dissolution of the binder. However, in this unsintered type electrode, the active material is fixed by the binder, whereby the binder remains among the particles of the active material. This deteriorates conductivity among the particles of the active material and between the active material and conductive plate. Therefore, this type of electrode is inferior in electrode characteristics.
On the other hand, the latter sintered type electrode does not entail the problem of lowering the conductivity since the active material is in direct contact with the sintered plaque and not through the binder. Further, this type of electrode has a conductive matrix present as the porous nickel sintered plaque, thereby having high conductivity within the electrode. Accordingly, the sintered type electrode has excellent electrode characteristics compared with the unsintered type electrode.
In this sintered type electrode having high conductivity, however, the active material becomes inactivated and undischargeable metallic cadmium accumulates with repetition of a charge and discharge cycle where cadmium hydroxide or cadmium oxide is employed as the active material. This problem is peculiar to the sintered type electrode and leads to lowering of the electrode capacity. In the unsintered type electrode having poor conductivity, the cycle life comes to an end because of another factor, for example, falling of the active material before the occurence of the above mentioned phenomenon, i.e. accumulation of the undischargeable metallic cadmium. To prevent this falling of the active material, methyl cellulose may be added to the electrode. However, the more methyl cellulose is added, the poorer conductivity the electrode has. This deteriorates the electrode characteristics.
The lowering of the electrode capacity with progress of the charge and discharge cycles is due to the following. That is, when the metallic cadmium is covered with fine cadmium hydroxide, it is difficult for hydroxide ions to be supplied to the active material from an electrolyte. This leads to the formation and accumulation of the undischargeable metallic cadmium. In order to prevent this phenomenon, it has been proposed to form a polymer film on surfaces of the cadmium active material (cf. Japanese Patent Publication Kokai No. 61-158,666) or to form a high polymer film on surfaces of the electrode (cf. Japanese Patent Publication Kokai No. 61-158,664).
These proposals intend to check the covering of the metallic cadmium with the inactive cadmium hydroxide by forming a high polymer film on the surfaces of the active material or of the electrode, controlling the hydroxide ion supply, thereby forming .gamma.-Cd(OH).sub.2 having a needle-like crystal structure. However, in these methods, access of oxygen gas to the active material during an oxygen gas consumption reaction is impeded as well. This affects the oxygen gas consumption reaction. This disadvantage is serious particularly in the case of the sintered type electrode having excellent oxygen gas consumption capability.
In addition, where the negative electrode is coated with a high polymer solution as disclosed in the above Japanese Patent Publication Kokai No. 61-158,664, the high polymer does not penetrate into pores of the negative electrode, whereby the high polymer adhesive cannot penetrate into surfaces of the active material within the electrode. Therefore, occulusion of the metallic cadmium cannot be prevented completely, and may hamper the contact between the metallic cadmium and the electrolyte. Accordingly, this method has not been sufficient to prevent the lowering of the negative electrode capacity.
Further, even in the method wherein the negative electrode is impregnated with the high polymer solution under a reduced pressure as disclosed in Japanese Patent Publication Kokai No. 61-158,666, the high polymer cannot penetrate into the pores of the negative electrode. Therefore, this method is also insufficient to prevent the lowering of the negative electrode capacity. In addition, this method requires equipment for reducing the pressure and a long time for vaccumizing. Consequently, this method entails a problem of high manufacturing cost of the negative electrode.
Next, as a further method, it has been proposed to add a polysaccharide or at least one of its derivatives to the active material. This method utilizes the property of the polysaccharide or its derivatives which reduces deposition nucleus of the cadmium hydroxide at the discharging time. Because of this property, refinement of the cadmium hydroxide particles is prevented, whereby it is checked that the metallic cadmium is covered with the cadmium hydroxide.
However, even with this negative cadmium electrode, utilization factor becomes lowered with progress of the charge and discharge cycles, which leads to the lowering of the negative electrode capacity. Therefore, satisfactory results cannot be obtained by adding the polysaccharide or at least one of its derivatives to the electrode. That is, where the polysaccharide mainly comprising a polysaccharide having a polymerization degree of less than 300 or its derivative is added to the electrode, the effect resulting from this addition becomes lower with repetition of the charge and discharge cycle over a long period. This is due to the fact that the polysaccharide or its derivatives having polymerization degree of less than 300 falls from the negative electrode because of their poor binding strength. Therefore, in order to maintain the effect for a long time, a large amount of polysaccharide or its derivatives must be added. However, this deteriorates the oxygen gas consumption capability.